The invention relates to a circuit arrangement for operating a discharge lamp, comprising a DC/AC converter provided with:
a series arrangement of a first and a second switching element between a first and a second input terminal for connection to a DC voltage source, which switching elements each have a control electrode and a main electrode, PA1 a load branch with at least a primary winding of a transformer, inductive means, and output terminals for connecting the lamp, a first end of said load branch being connected to a junction point situated in the series arrangement and a second end to an input terminal, PA1 a first and a second secondary winding of the transformer between the control electrode of the first switching element and the main electrode of the second switching element, PA1 a starting circuit with first resistive means between the first input terminal and the control electrode of the first switching element, and with first capacitive means connected in series with the first secondary winding between the control electrode and the main electrode of the first switching element.
Such a circuit arrangement is known from U.S. Pat. No. 4,748,383. The starter circuit initiates an oscillation of the DC/AC converter after switching-on of the circuit arrangement.
The secondary windings of the transformer each have a comparatively large number of turns compared with the primary winding, and a series arrangement of two zener diodes connected in mutually opposed directions and having a comparatively low breakdown voltage is included between the control electrode and the main electrode of each switching element. The voltage between the control electrode and the main electrode, called control voltage hereinafter, as a result has a substantially square-wave characteristic. The switching time which elapses between the moment the control voltage has a zero passage and the moment the control voltage exceeds the threshold voltage, i.e. the voltage at which the switching element becomes conductive is short as a result. It is realized thereby that the switching elements are in the conductive state for approximately the same duration.
It is a disadvantage in the known circuit arrangement that comparatively much power is dissipated in the zener diodes which limit the control voltage of the switching elements. Not only does this detract from the efficiency of the switching arrangement, but the accompanying heat generation also hampers a miniaturization of the circuit arrangement. If the secondary windings of the coil have comparatively few turns, the control voltage need not be limited, so that zener diodes may be omitted or serve as a protection only during lamp ignition. The power dissipation in the zener diodes is negligibly small then. The gradient of the control voltage, however, is much more gradual then. Comparatively small differences in the threshold voltage of the switching elements may then lead to comparatively major differences in the time of conduction, i.e. the duration of the conductive state. The result of this is that the power dissipation in the switching elements increases.